Recently, imide-based polymers have been used in surface-stabilized films on the exposed terminals of PN junction, interlayer insulating films of multi-layer wiring, .alpha.-ray shield films for preventing malfunctions of memory elements due to radiations (particularly, .alpha.-rays), flexible print circuit substrates, etc., but the ordinary polyimides have a high moisture absorbability in these applications and have various troubles due to this property. For example, an increase in leak current, corrosion of wiring materials (particularly in the case of aluminum), etc. are the problems. Furthermore, the moisture-absorbed polymers have problems such as swelling, when rapidly heated in the steps of gold wire bonding, solder reflow, etc.
As a result of studies of heat resistance and moisture absorbability of polymers having various chemical structures, the present inventors have found that polymers of low moisture absorbability generally have a poor heat resistance. For example, in comparison with polyimides having repetition units of the following formula (A), polyimides having repetition units of the following formula (B) have a moisture absorbability reduced to about 1/3, but have a lower thermal decomposition temperature even by 40.degree. C. ##STR2##
Likewise, polymers having repetition units of the following formula (C) have a moisture absorbability reduced to about 1/4 and a lower thermal decomposition temperature by about 100.degree. C., as compared with the polyimides of formula (A). ##STR3##
An object of the present invention is to provide novel polyimides having a low moisture absorbability and a distinguished heat resistance and being particularly useful for applications as various insulating films in electronics, and also provide polyamide acids as raw materials for them.